Polytetrafluoroethylene (hereinafter referred to as "PTFE") powders used generally are roughly classified into two kinds of powders, namely a powder for molding obtained by aqueous suspension polymerization substantially in the absence of emulsifying agent (so-called "molding powder") used mainly for compression molding and ram extrusion molding and a powder obtained by emulsion polymerization in the presence of emulsifying agent and so-called "fine powder" used mainly for paste extrusion molding. The latter emulsion-polymerized fine powder can be used for molding as it is without being pulverized since it is a fine particle. To the contrary, the former molding powder is finely pulverized and used as a powder for molding since coarse particles obtained by aqueous suspension polymerization usually has a large particle size of several millimeters.
Among those powders, the present invention relates to the former molding powder, namely PTFE molding powder obtained by aqueous suspension polymerization. The PTFE molding powder has been prepared, in general, through the following steps.
Step (1): PTFE coarse particles having a particle size of several millimeters are prepared by aqueous suspension polymerization of tetrafluoroethylene. PA1 Step (2): The obtained PTFE coarse particles are pulverized in water medium to give roughly pulverized PTFE particles having a particle size exceeding about 100 .mu.m. PA1 Step (3): The resulting roughly pulverized PTFE particles are washed in a washing tank. PA1 Step (4): Then the washed roughly pulverized PTFE particles are dried. PA1 Step (5): Lastly the dried roughly pulverized PTFE particles are finely pulverized into fine particles having a particle size of not more than 100 .mu.m, particularly not more than 50 .mu.m, thereby a PTFE molding powder is produced. PA1 Step (6): Mechanical force such as stirring is applied to the PTFE molding powder obtained in the step (5), in the presence or absence of water in the presence of an organic liquid being capable of wetting PTFE, thus executing agglomeration granulation of the PTFE molding powder. PA1 Step (7): The agglomeration-granulated PTFE molding powder is dried. An average particle size of the agglomeration-granulated powder is usually not less than about 100 .mu.m.
However, in the above-mentioned process, a small amount of impurities such as a residual monomer or catalyst contained in the roughly pulverized PTFE particles cannot be removed efficiently through the washing of the step (3). This is presumed to be one of factors for endowing the resulting PTFE molding powder with an adverse effect on physical properties. For that reason, there are proposed attempts of increasing washing efficiency by increasing the number of washing steps or by changing washing conditions such as temperature and rotation speed. However the particle size of the roughly pulverized particles usually exceeds about 100 .mu.m and is still large. Therefore, even by such attempts, it is difficult to remove the impurities from the particles sufficiently. Though such a problem with respect to the washing may be improved by re-washing after the fine pulverization of the step (5) and then drying, it results in two drying steps and is not allowable in view of efficient process steps.
Also in order to give a high apparent specific gravity and excellent flowability to the PTFE molding powder obtained by aqueous suspension-polymerization and then fine pulverization, there is also known an agglomeration-granulation process. The agglomeration-granulated powder is available as a molding powder suitable for automatic molding process from the viewpoint of its good flowability. The granular powder through agglomeration is prepared in general by the following steps.
In the step (6), there are known that mechanical force such as stirring is applied in the presence of water or in the absence of water. Though either of them can be employed, when the force is applied in the presence of water, controlling of the step is easier, a powder having uniform particle size can easily be obtained and it is advantageous for automation of the step. Thus many industrial processes employ the agglomeration-granulation process in the presence of water.
However, in case of the presence of water, since the powder dried in the step (4) is wetted in the step (6) and then dried again (in the step (7)), the number of steps increases and as a result, cost becomes high.
An object of the present invention is to provide a process for preparing a PTFE molding powder which is capable of decreasing efficiently an amount of impurities remaining therein.
Another object of the present invention is to provide a process for preparing an agglomeration-granulated PTFE molding powder with decreased number of steps.